What really killed off the dinosaurs?

Just as scientists thought they had nailed down the answer, the debate has been reopened. A team of scientists claims the widely accepted theory that the extinction was triggered by a huge asteroid thumping into Mexico 65m years ago, cannot be true.

Evidence that a giant asteroid impact was the cause of the dinosaurs' demise first emerged in the 1980s. Scientists analysing ancient soils in Italy found that layers of clay from the end of the Cretaceous period, the time the dinosaurs vanished, were unusually rich in a heavy metal called iridium. Later evidence of the layer was found in other countries, including Denmark and New Zealand. The most likely cause was believed to be an extraterrestrial rock that struck Earth and showered iridium across the continents. Such an impact would have had a devastating affect on life, as hot rocks fell from the skies and dust shrouded the sun.

The theory gained credibility a decade ago when scientists declared they had found the smoking gun for the impact. A crater more than 100km across, that seemed to date back to the end of the Cretaceous period, was discovered near a village called Chicxulub on the Yucatan peninsula.

But according to Gerta Keller, a geologist at Princeton University, the Chicxulub crater is not linked.

Keller's team analysed rock which had melted in the intense heat of the impact, been thrown into the stratosphere and scattered far and wide. They found the oldest pieces, which have the same chemical composition as molten rock in the crater, were formed some 300,000 years before the dinosaurs became extinct. Samples from the crater back up the idea that dinosaur life existed long after the impact at Chicxulub, says Keller.

"What this means is that Chicxulub is not the smoking gun that caused the extinction. What really killed the dinosaurs must have been another impact," she says.

And so the search for the real smoking gun is on again. If, of course, an asteroid was actually responsible.